1. Field of the Invention
The present invention relates to both a composite material and a method for producing the composite material. More particularly, the invention relates to both a pressure-bonded stainless steel and carbon steel composite material, and a method for producing the composite material.
2. Description of the Invention Background
Pressure-bonded composites of individual layers of steel are used in a variety of applications. As noted in U.S. Pat. No. 3,693,242, incorporated herein by reference, a composite comprising a core layer of plain carbon steel containing a carbide former and outer layers of stainless steel is used in producing certain metal items such as, for example, cookware. Related U.S. Patents include U.S. Pat. Nos. 3,795,971, 3,862,484, 3,944,396 and 3,956,809, which patents are hereby incorporated by reference. The stainless steel surface layers of the composite are corrosion-resistant and provide an attractive appearance, while the plain carbon steel core layer has relatively greater heat conducting properties than the stainless steel layers. A specific example of this type of composite material is A.I.S.I. Type 304DA ("T-304DA") which has a conventional low carbon steel core material and surface layers of A.I.S.I. Type 304 stainless steel ("T-304"). In hot processing conventional T-304DA, the standard practice is to cool the hot-rolled band to below 900.degree. F. on the hot run out table before coiling. Failure to so cool the hot-rolled band to below 900.degree. F. (482.degree. C.) causes carbon from the low carbon steel core material to migrate into the interface region between the stainless steel surface layers and the low carbon steel core. This carbon migration occurs against the carbon concentration gradient and is dependent primarily on the attainment of an overall lower free energy. Such a migration against the concentration gradient frequently occurs from plain carbon steel to stainless steel because the stainless steel contains chromium which attracts the carbon.
Once migrated to the interface region, the carbon forms carbides, and thereby results in carbide enriched regions near the interface region. These carbide enriched regions cause so-called "Hook Cracks" upon drawing, an undesirable defect in the final product. In addition, carbon migration leads to the formation of a coarse ferrite grain layer in the low carbon core material which causes the defect known as "Orange Peel Surface" in the final product. Also, failure to cool below 900.degree. F. may result in metal separation, whereby one or more stainless steel layers delaminate from the low carbon steel core layer.
In preventing these problems by cooling the hot-rolled band to below 900.degree. F., other problems arise. Rapid cooling to below 900.degree. F. on the hot run out table is disadvantageous because coiling the hot-rolled band after rapid cooling on the hot run out table, by, for example, water quenching, results in mechanical defects on the coil surface. These defects must be removed by surface grinding, necessitating additional production cost. Higher coiling temperatures would result in fewer mechanical defects.
One approach to the aforementioned problems is to cool hot-rolled bands of stainless steel/carbon steel composite material so that the coiling temperature is greater than 900.degree. F., but less than 1200.degree. F. (649.degree. C.). This practice would minimize the propensity for both carbide formation in the interface region and the formation of coarse ferrite grains in the core material, while minimizing the prevalence of the mechanical defects on the coil surface which are associated with rapid cooling. Above about 1200.degree. F., the defects associated with carbide formation could no longer be tolerated. However, because this approach is a compromise, it does not provide an entirely satisfactory solution; the aforementioned enriched carbide regions and coarse ferrite grains still form to some extent.
An article entitled "Development of New Low Carbon Low Alloy Steel Suited to be Clad with Stainless Steel" by Hashimoto et al., ISIJ, Vol. 31 (1991), pp. 706-711 discloses stabilized steels having less than 0.01% carbon and containing columbium (niobium) and titanium. The ferrite grain size is controlled by controlling the final anneal temperature and using faster cooling rates.
In addition to the above disadvantages experienced with the hot rolling of stainless steel/carbon steel composite materials, drawbacks are also experienced during processing. Stainless steel/carbon steel composite materials may require annealing at temperatures greater than 1700.degree. F. (927.degree. C.) in order to release stresses built up during the bonding process and any subsequent cold forming, and to dissolve chromium carbides formed in the stainless steel layers. Typical carbon steels, such as type 1006, used as composite core material, transform to a predominantly austenitic structure at temperatures below 1700.degree. F., temperatures below the recrystallization temperature of typical stainless steels. Therefore, annealing at temperatures significantly greater than 1700.degree. F. may cause significant coarsening of the austenite grains in the core layer, which transform into equally coarse ferrite grains upon cooling.
Accordingly, it is an object of the present invention to provide a carbon steel/stainless steel pressure-bonded composite material which may be coiled without purposeful cooling at a coiling temperature greater than 1200.degree. F. with a reduced propensity for the formation of both (i) carbide enriched areas in the interface region and (ii) coarse ferrite grain layers in the carbon steel core material.
It is an additional object of the present invention to provide a carbon steel/stainless steel pressure-bonded composite material which has a substantially reduced incidence of mechanical defects after cooling at coiling temperatures greater than 1200.degree. F., preferably greater than 1400.degree. F. to about 1450.degree. F. (760.degree.-788.degree. C.), as compared with T-304DA having conventional carbon steel core material.
It is also an object of the invention to provide a T-304DA composite material which may be processed using high anneal temperatures, preferably about 1850.degree. F. up to about 1950.degree. F., during the hot band anneal procedure without a significant amount of grain coarsening in the austenitic phase.
It is also an object of the invention to provide a stainless steel/carbon steel composite, such as T-304DA composite material, which satisfies the above objectives and which has mechanical properties comparable to those composites having conventional carbon steel core material.
In addition, it is an object of the invention to eliminate temper rolling as a final processing step.